Commercially available reflective type liquid crystal displays generally comprise top and bottom glass plates and liquid crystal material sandwiched between the plates. A polarizer is placed on the outer surface of each of the glass plates, the polarizers being so oriented that their axes of polarization are orthogonal, and a reflecting surface is placed behind the bottom polarizer. In the absence of an electric field, the liquid crystal material typically serves to rotate by 90.degree. the polarization state of light passing through it with the result that the light is transmitted through the bottom polarizer whereupon it is reflected by the reflecting element back to the observer. When the electric field is applied, the polarization state of the light is unaltered by the liquid crystal material with the consequence that no light passes through the bottom polarizer and hence no light is reflected back to the observer.
In order to display a symbol such as a digit for a clock, transparent planar electrodes are formed on the inner surfaces of the top and bottom plates. Typically, a standard seven segment electrode for each digit is formed on the inner surface of the top plate, and several multiplexed electrodes which collectively overlap the segmented electrodes are formed on the bottom plate.
The correct time is displayed by applying an electric field across selected portions of the liquid crystal material via the appropriate segmented electrodes and multiplexed electrodes. Light passing through those portions of the liquid crystal material under the influence of the electric field is not transmitted by the second polarizer and hence those portions appear dark, while all other areas transmit light to the observer and hence appear bright. A display is thereby effected corresponding in shape to those segmented electrodes to which the electric field is applied, the display appearing dark on a bright background. In the case of a digital time display, a microprocessor controls the application of the electric field to the electrode segments for displaying the correct time.
Commercially available transmissive-type liquid crystal displays operate in a similar manner, the difference typically being that in transmissive-type displays the reflecting element is eliminated, the polarization axes of the polarizers are parallel, and a light source is disposed behind the bottom polarizer. The displayed symbol, which is observed through the top polarizer, appears bright against a dark background.
It will be appreciated that conventional liquid crystal displays require electrodes in the shape of the symbol to be displayed. Consequently, such conventional displays are highly uneconomical where there is a need for only a limited number of displays of a particular symbol. Furthermore, conventional displays offer no possibility of varying the displayed symbol, except in a limited fashion by selective energization of the electrodes, as in the time display discussed above.
It is therefore desirable to provide an alternative display which overcomes the deficiencies of the prior art displays.